Colorectal cancer is a significant cancer burden to the general population of many developed countries. In the United States alone, there are over 130,000 new cases of colorectal cancer per year, and over 65,000 deaths per year resulting from colorectal cancer. Colorectal cancer it is second only to lung cancer in cancer morbidity in the United States.
The progression of colorectal cancer, or colorectal tumorigenesis, is a multi-step process involving the loss of function of so-called tumor suppressor genes, as well as the activation of oncogenes. Fearon et al., Cell 61: 759-67 (1990); Paraskeva et al., Anticancer Research 10: 1189-200 (1990). It is also marked by several phenotypically distinct stages during progression. These include normal, hyperplastic, benign, carcinoma and metastatic stages. These distinct stages make colorectal cancer an exceptionally useful paradigm for the studying the molecular genetic basis of cancer in general.
Among the classical oncogenes implicated in cancer, the ras and myc genes have been found to be activated and/or show elevated expression in colorectal tumors. About half of large adenomas and at least half of carcinomas contain activated K-ras genes. Forrester et al., Nature 327: 298-303 (1987); Bos et al., Nature 327: 293-97 (1987); Burmer et al., Proc. Nat'l Acad. Sci. USA 86: 2403-07 (1989). C-myc over expression and occasional gene amplification have also been demonstrated in colorectal tumors. Erisman et al., Mol. Cell. Biol. 5: 1969-76 (1985); Imaseki et al., Cancer 64: 704-09; Finley et al., Oncogene 4: 963-71 (1989). Furthermore, deregulated c-myc expression can be suppressed by microcell-mediated transfer of chromosome 5, which is the locus for the putative tumor-suppressor genes, APC (for adenomatous polyposis coli) and MCC (for mutated in colorectal carcinoma) discussed below. Rodriguez-Alfageme et al., Proc. Nat'l Acad. Sci. USA 89: 1482-86 (1992). Although the importance of oncogenes in cancer development can not be ignored, it is the presently the tumor suppressor genes which have drawn the most interest for study of cancer development.
Several tumor suppressor genes have been implicated in colorectal tumor progression. One of the more noteworthy tumor suppressor genes is p53. This gene has a locus at chromosome band 17p13 and is lost in a large majority of colon carcinomas (though not as much in adenomas). Often the lesion, which refers to genetic mutations, consists of a deletion of one allele and a point mutation at one of several hotspots in the remaining allele. Baker et al., Science 244: 217-21 (1989), Nigro et al., Nature 342: 705-07 (1989). Importantly, it has been shown that transfection of a wild-type p53 gene into colon cancer cell lines in vitro results in a suppression of cell growth, thereby demonstrating that the p53 gene product, a tumor suppressor, has a direct effect on one major cancer characteristic. Baker et al., Science 249: 912-15 (1990).
Genes APC and MCC identified above have been mapped to a locus at chromosome band 5q21. Groden et al., Cell 66: 589-600 (1991); Kinzler et al., Science 253: 661-64 (1991); Kinzler et al., Science 251: 1366-70 (1991). This is the site which is linked to the inherited disorder adenomatous polyposis coli, which is a disorder marked by multiple polyposis and a very high incidence of colon carcinoma at an early age. Both genes contain mutations and/or deletions in colon carcinoma, however, MCC mutations are not common among tumors, whereas the APC lesions are more common and found in the germ line genomic DNA of APC patients. Kinzler et al., Science 251: 1366-70 (1991); Nishisho et al., Science 253: 665-69. It is notable that transfer of chromosome 5 to colon cancer cells lacking a normal APC gene suppresses tumorigenicity. Goyette et al., Mol. Cell. Biol. 12: 1387-95 (1992). This reinforces the concept that APC and/or MCC are tumor suppressor genes. Another gene, the DCC gene (for deleted in colorectal carcinoma), is located at chromosome band 18q21 and also is lost in a large majority of colon carcinomas and about fifty percent of late adenomas. A portion of the DCC gene bears a homology to the neural cell adhesion molecule (N-CAM). Fearon et al., Science 247: 49-56 (1990). This suggests that the DCC gene product may play a role in cell-to-cell contacts. A specific role in colorectal tumor progression, however, has not been ascertained.
The identification of such genes, the absence or impairment of which is linked to cancer, yields insights into the initiation and progression of cancer and other abnormalities. Additionally, the existence of such genes raises the possibility that other tumor suppressor genes may exist.